Optical & Photonic RF

DBR Laser

Q: How does a DBR laser differ from a DFB laser?

In a DFB (distributed feedback) laser the Bragg grating runs continuously along the active, current-pumped gain region, so the grating and gain overlap everywhere. In a DBR laser the grating sits in a separate passive section at one or both ends of the cavity, physically distinct from the gain region. This separation lets the DBR grating section be biased independently: injecting current changes the carrier density and refractive index of the passive section, which shifts the Bragg wavelength and tunes the laser without modulating the gain. The trade-off is that the passive-active interface introduces a small reflection and mode-partition risk, so DFB lasers typically achieve more robust single-mode yield, while DBR lasers offer wider electronic tuning, often 8 to 15 nm versus under 3 nm for a simple DFB.

Q: How is a DBR laser tuned electronically?

A three-section DBR laser has independent contacts for the gain section, a phase-control section, and the Bragg grating section. Current injected into the grating section raises the carrier density, which lowers the refractive index via the plasma effect and shifts the Bragg wavelength toward shorter wavelengths. The phase section is adjusted to keep a cavity mode aligned with the grating reflection peak, producing continuous (mode-hop-free) tuning over a few nanometers, with discrete mode hops extending the total range to 8 to 15 nm. Tuning speed is set by carrier dynamics, allowing nanosecond-scale wavelength switching, far faster than the thermal tuning used in many other single-mode sources.

Q: What linewidth and side-mode suppression can a DBR laser achieve?

Commercial DBR lasers reach side-mode suppression ratios (SMSR) of 35 to 50 dB, ensuring clean single-mode emission. Intrinsic Lorentzian linewidths are typically 100 kHz to a few MHz for a standard three-section InP DBR; adding a long, low-coupling external or sampled-grating section can narrow this below 100 kHz. For photonic RF and radio-over-fiber links the relevant figure is relative intensity noise (RIN), usually under -150 dBc/Hz, and phase noise that translates directly into the noise floor of the recovered microwave carrier. Linewidth matters most in coherent and optical heterodyne systems where two laser phases beat together to generate the RF tone.

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Short for distributed Bragg reflector laser, this semiconductor source places a periodic corrugated grating in a passive waveguide section outside the current-pumped gain region, where it acts as a sharply wavelength-selective mirror. The grating reflects only the band of wavelengths satisfying the Bragg condition, forcing the cavity into a single longitudinal mode with side-mode suppression of 35 to 50 dB. Unlike a DFB laser, the grating and gain are physically separate, so a current applied to the passive Bragg section shifts the reflection peak and tunes the emission wavelength electronically across 8 to 15 nm. The resulting narrow-linewidth, mode-hop-free output drives radio-over-fiber transport, optical heterodyne generation of microwave carriers, and coherent links where laser phase noise sets the recovered RF noise floor.

Category: Optical & Photonic RF

Tuning Range: 8 to 15 nm

SMSR: 35 to 50 dB

How the Bragg Grating Enforces Single-Mode Emission

A Fabry-Perot semiconductor laser oscillates on many longitudinal modes because its cleaved-facet mirrors reflect almost equally across a wide band. The distributed Bragg reflector replaces one or both of those broadband facets with a periodic refractive-index grating etched into a passive waveguide. Light experiences a strong distributed reflection only when its wavelength satisfies the Bragg condition, where the round-trip phase per grating period adds in phase. The reflectivity peak is narrow, on the order of a fraction of a nanometer, so only one cavity mode falls under the peak and lases. Because the grating sits outside the gain region, it is not pumped and does not generate carriers, which keeps the reflection peak stable against the gain-region carrier fluctuations that broaden linewidth.

The defining advantage of the DBR architecture is the electrical separation of grating, phase, and gain functions. In a three-section device, the gain section provides optical amplification, the phase section sets the round-trip phase so a cavity mode aligns with the Bragg peak, and the grating section selects the wavelength. Injecting current into the passive Bragg section reduces its refractive index through the free-carrier plasma effect, which shortens the local optical period and shifts the Bragg wavelength toward the blue. This is the mechanism behind fast electronic tuning, with switching times in the nanosecond range rather than the milliseconds typical of thermal tuning.

The cost of that flexibility is the passive-active interface. The junction between pumped and unpumped material introduces a small parasitic reflection and a region of differing optical loss, which can promote mode partition noise and slightly degrade single-mode robustness compared with a continuously corrugated DFB device. Designers manage this with anti-reflection-tapered transitions, sampled or superstructure gratings for extended tuning, and careful phase-section control to suppress mode hops across the tuning curve.

Governing Relationships

Bragg Wavelength:
λ_B = 2 × n_eff × Λ / m

Carrier-Induced Wavelength Shift (tuning):
Δλ / λ_B ≈ Δn_eff / n_eff

Grating Stopband (coupled-mode):
Δλ_stop ≈ λ_B² × κ / (π × n_g)

Where n_eff = effective waveguide index, Λ = grating period (≈ 240 nm for λ_B ≈ 1550 nm at m = 1), m = grating order, κ = coupling coefficient, n_g = group index. Example: Λ = 243 nm, n_eff = 3.19, m = 1 → λ_B ≈ 1551 nm. A 1% index change in the Bragg section tunes λ_B by roughly 15 nm.

Single-Mode Laser Source Comparison

Source	Grating vs. Gain	Tuning Range	Tuning Speed	Typical Linewidth	Best RF/Photonic Use
DBR (3-section)	Separate passive section	8 to 15 nm	ns (carrier)	0.1 to 2 MHz	Fast-switched WDM, RoF
DFB	Continuous, over gain	2 to 3 nm	ms (thermal)	0.1 to 1 MHz	Fixed-channel transmitters
Sampled-grating DBR	Two Vernier gratings	40 to 50 nm	ns (carrier)	1 to 10 MHz	Wideband tunable WDM
External-cavity	External grating/mirror	40 to 100 nm	ms (mechanical)	1 to 100 kHz	Coherent, narrow-linewidth
VCSEL	Vertical DBR mirror stacks	< 1 to 10 nm (MEMS)	µs to ms	10 to 100 MHz	Short-reach, low-cost links

Common Questions

Frequently Asked Questions

How does a DBR laser differ from a DFB laser?

In a DFB device the Bragg grating runs continuously along the pumped gain region, so grating and gain overlap. A DBR places the grating in a separate passive section that can be biased independently: current there shifts the Bragg wavelength and tunes the laser without touching the gain. DFB lasers give more robust single-mode yield; DBR lasers give wider electronic tuning, often 8 to 15 nm versus under 3 nm for a simple DFB.

How is a DBR laser tuned electronically?

A three-section DBR has separate gain, phase, and Bragg contacts. Current in the Bragg section lowers the index via the plasma effect, shifting λ_B toward shorter wavelengths; the phase section keeps a cavity mode aligned for mode-hop-free tuning over a few nm, with mode hops extending the range to 8 to 15 nm. Carrier-driven tuning switches wavelength in nanoseconds, far faster than thermal tuning.

What linewidth and side-mode suppression can a DBR laser achieve?

Typical SMSR is 35 to 50 dB. Intrinsic Lorentzian linewidth runs 100 kHz to a few MHz for a standard InP three-section DBR, narrowing below 100 kHz with long low-κ gratings. RIN below -150 dBc/Hz and the laser phase noise both matter for photonic RF, since two beating laser phases set the noise floor of any heterodyne-generated microwave carrier.

Related Terms

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